The present invention relates to an optical disk which can increase its recording density and a manufacturing method thereof.
A conventional optical disk, e.g. DVD (Digital Versatile Disc) is formed such that, as its cross section is schematically shown in FIG. 6, a reflection film 102 having a thickness of several tens nanometers, e.g. thickness of 50 nm is formed on a light transmissive disk substrate 101 having a signal recording portion 100 on which successive pits are formed and a protection layer 103 made of an organic material having a thickness of, e.g. about 10 xcexcm, covers the surface of the reflection film.
To read out a signal from this DVD, reproducing laser light 105 is irradiated on the signal recording portion through an objective lens 104 from the side of the light transmissive disk substrate 101, and pits in the signal recording portion 100, is detected i.e. recorded data is read out by returned light of the reproducing laser light.
In the case of ordinary DVD, since the disk substrate has a thickness of 0.6 mm and a signal is reproduced through this disk substrate 101, the numerical aperture N. A. of an objective lens that a reproducing pickup includes is restricted to about 0.6.
By the way, the size of a-spot of reproducing light is in proportion to a ratio xcex/N. As between a wavelength (of the reproducing laser light 105 and an N. A. of the objective lens 104. In the conventional ordinary DVD, the wavelength of the reproducing light is 650 nm; the N. A. is 0.6; and one side of the disk has a recording capacity of 4.7 GB.
For example, consider an optical disk which is reproduced by reproducing laser light having a wavelength xcex of 400 nm through an objective lens having a numerical aperture N. A. of 0.85. Then, the recording capacity of one side of this optical disk can simply be calculated to be 25 GB based on the proportion of this disk to the above-mentioned conventional DVD.
However, the recording capacity thus calculated is obtained in consideration of only the characteristics of the reproducing pickup. In actual practice, the pit size of the optical disk should also be made minute and made with high accuracy.
An ordinary optical disk manufacturing method is as follows. As shown in FIG. 7, on a glass disk 106 having a diameter of about 200 mm and a thickness of several millimeters and whose surface was precisely polished, there is formed a photoresist layer 107 having a film thickness of about 0.1 xcexcm, in which a photoresist sufficiently sensitive to a wavelength of a recording laser light source 107 of a laser cutting apparatus is uniformly spin-coated.
This photoresist layer 107 is subjected to exposure. When this exposure is carried out, there is employed a pattern exposure in which laser light 109 having a wavelength of 413 nm from a recording laser light source 108 made of, e,g, a Kr laser, is on/off-modulated by an acousto-optic modulator, i.e. AOM 110 in response to a recording signal and focused to irradiate on the photoresist layer 107 through an expander 111 and an objective lens 112, making this laser light spot scan the photoresist layer 107 in a spiral fashion to form latent images of pits and grooves.
Thereafter, when this photoresist layer 107 is developed by an alkaline developer, the exposed portion is dissolved, and there is formed, as shown in FIG. 8, a master disk 121 in which an uneven pattern 120 comprising pits and grooves is formed on the photoresist layer 107 coated on the disk 106.
Then, on this uneven pattern 120 of this master disk 121, as shown in FIG. 8, there is deposited a metal layer 122 having a thickness of about 300 (m by sequentially effecting nonelectrolytic plating and electroplating of nickel (Ni) in such a manner as to fill this uneven pattern. Thereafter, this metal layer 122 is peeled from the master disk 121, and a stamper 123 having an inverted version of the uneven pattern 120 of the master disk 121 is obtained from the metal layer 122 thus peeled.
This stamper 123 is disposed within, e,g, an injection molding die for injection molding to produce the optical disk substrate 101 made of polycarbonate (PC) or the like, as shown in FIG. 9.
On this optical disk substrate 101, there are transferred uneven patterns of the stamper 123, i.e. there are formed pits and grooves corresponding to the uneven pattern of the master disk, thus causing the signal recording portion 100 shown in FIG. 6 to be formed.
In this optical disk substrate 101 on its surface that the signal recording portion 100 is formed, there is deposited, as shown in FIG. 9, the reflection film 102 shown in FIG. 6 by sputtering using aluminum (Al) target 124 for example. Further, the protection film 103 is formed on this reflection film.
This protection film 103 is generally made of a ultraviolet-curing resin cured by irradiating ultraviolet rays after the ultraviolet-curing resin is applied to the reflection film 102 by spin-coating so as to have a uniform thickness.
Since the limit of the numerical aperture of the objective lens 112 is generally about 0.9, the optical disk thus obtained after the master disk was produced by the pattern exposure effected by laser light having a wavelength of 413 nm will have successive pits formed thereon, which have the shortest pit length of 0.4 xcexcm and the track pitch of 0.74 xcexcm. In addition, the width of the pit, i.e. the length along the radial direction of the disk is about 0.35 xcexcm which is half of the track pitch.
Due to such restrictions imposed upon the pit size when it is made minute and with high precision, the pit size cannot be made so minute and made with high such precision that an optical disk having a recording capacity of, e,g, 15 GB or more, in particular 25 GB can be obtained by the conventional pattern exposure using the laser light having the wavelength of 413 nm.
The present invention provides an optical disk and a method of manufacturing the same which can provide a high-density recording, i.e. which can provide the aforementioned recording capacity of, e.g. 15 GB or higher recording capacity of, e.g. 25 GB.
An optical disk according to the present invention may comprises an optical disk substrate in which successive pits corresponding to a recording signal are formed, a reflection film formed in this optical disk substrate on its surface that the successive pits are formed and a light transmissive layer formed on this reflection film.
When a recorded signal is read out, i.e. reproduced from this optical disk, a signal recorded as the successive pits is read out from the optical disk by irradiating short-wavelength laser light having a wavelength of 350 nm to 420 nm from the side of the light transmissive layer formed on the surface of the optical disk.
Moreover, when this optical disk is seen from the side of the light transmissive layer irradiated by reproducing laser light, the successive pits contain pits having length and width ranging from 80 nm to 250 nm, and the thickness of the reflection film is selected to be 20 nm or less, e.g. 8 nm greater.
The optical disk manufacturing method according to the present invention is the manufacturing method of producing the above described optical disk according to the present invention, comprising the steps of producing an optical disk manufacturing master disk for forming successive pits using laser light having a wavelength ranging from 200 nm to 370 nm for exposure in response to a recording signal, producing an optical disk substrate in which the successive pits containing pits having length and width both ranging from 80 nm to 250 nm are formed by transferring the successive pits of this master disk and forming a reflection film having a film thickness of 20 nm or less in this optical disk substrate an its surface that the successive pits are formed.